Speakerphones or hands-free telephones include a loudspeaker associated with the instrument which transmits into the vicinity of the telephone instrument an audio signal representing the sound from a telephone at the other end of the telephone wire, and also includes a microphone which picks up sounds produced by a local speaker for transmission to a remote telephone. Ordinarily, a speech detector selects either the loudspeaker mode of operation or the microphone mode of operation, so as to avoid creation of echoes on the line due to the microphone picking up the sound generated by the loudspeaker.
It has been noticed that the sound produced at the far end of a telephone line in response to the sound picked up by the microphone of a speakerphone has an undesirable reverberation, such that the sound seems to come from the inside of a "rain barrel". This rain barrel effect is caused by single or multiple echoes of the voice of the local speaker from the walls of the room in which the sending end speakerphone is located. It is well known in the telephone art to use cancellation techniques to suppress echoes in the electrical transmission which arise due to multiple transmission paths or loops attributable to insufficient isolation in coupling hybrids as described for example in the paper Echo Cancellation in Speech and Data Transmission, by D. Messerschmitt, published at pp. 283-297 of the IEEE Journal on Selected Areas in Communication, VOL. SAC-2, No. 2, March 1984. However, the "rain barrel" effect referred to above occurs in the signals entering the microphone, as opposed to problems in the electrical transmission path. Thus, the solutions mentioned in the Messerschmitt paper have no applicability to the problem to which the invention is directed.
U.S. Pat. No. 4,118,601 issued Oct. 3, 1978, to Yeap describes a scheme for interposing a bank of filters and attenuators in the sound path of a audio loudspeaker sound system located in a room and to use white noise from a noise generator to set the equalizer to produce a substantially flat frequency response in spite of the room echoes. Such a scheme is directed towards the generation of a substantially constant response as a function of frequency of the audio portion of a system when driven by a substantially constant amplitude electrical signal. However, Yeap does not suggest how to generate an electrical signal corrected for echoes in the generating audio signal.
It would be desirable to generate an electrical signal representative of an audio signal in such a fashion that the electrical signal does not include portions representative of echoes occurring in the room in which audio signals were generated.